This invention relates to an apparatus for filtering high temperature gases, such as high pressure gases discharged from a circulating fluidized bed reactor. The filtration apparatus typically comprises porous ceramic filter elements. The present invention filters high temperature gases containing solid contaminants, efficiently removing accumulated solids from the filtering elements in a safe manner.
It is known in the prior art to use ceramic filters in order to remove particulates from hot gas streams. It is e.g. known to use candle type ceramic filters (as shown in U.S. Pat. No. 4,869,207) supported by a tube sheet for cleaning hot gases. The size of the filter housing is however presently limited, the practical limit for the diameter of a pressure vessel with candle type filters being about 3-5 m.
It has also been suggested to utilize monolithic ceramic filters, having e.g. a plurality of passageways extending longitudinally from inlet end to outlet end, but being plugged to prevent direct passage of the feed stock through the passageways from the inlet face to the outlet face. The cleaning capacity of clusters of such monolithic ceramic filters is much higher than of conventional candle type or tubular ceramic filters. The monolithic ceramic filters thereby being less space consuming than conventional tubular or candle type ceramic filters. The mounting of these elements in filtration vessels in high temperature surroundings with possible large temperature variations has however led to very complicated constructions.
It is also known that it is essential to have the filters cleaned, e.g. after certain pre-determined operating periods, in order to be able to maintain the desired pressure drop. Commonly used methods of cleaning the filter employ a reverse directional pulse of gas for flushing the filter. This kind of method is disclosed e.g. in U.S. Pat. No. 5,284,498 showing in a filtration vessel a single integral inner shell provided with ceramic monolithic filter elements for passing clean gas from the inner dirty gas side of the inner shell to the outer annular clean gas side of the shell. A backpulsing nozzle for backpulsing cleaning gas is provided in front of each monolithic filter element.
According to one embodiment of the present invention, an apparatus for filtering high temperature gases both from pressurized (i.e. superatmospheric pressure, typically over 2 bar) systems and atmospheric systems, is provided. The filtration apparatus comprises following elements: A generally upright vessel having a top, a bottom and a side wall. At least one generally horizontal filter supporting plate, e.g. a tube sheet like partition, adjacent the top or the bottom of the vessel, the supporting plate dividing the vessel to provide a dirty gas side and a clean gas side. A high temperature dirty gas inlet disposed in the side wall on the dirty gas side of the at least one filter supporting plate. At least one clean gas outlet disposed adjacent the top or the bottom of the vessel on the clean gas side of the at least one filter supporting plate. At least one generally upright hollow chamber element, having an interior volume and one closed end and one open end, disposed on the dirty gas side of the at least one supporting plate and connected at its open end to an opening in the supporting element, for connecting the interior volume of the hollow chamber element with the clean gas side of the supporting plate. The hollow chamber element having at least one generally impervious side wall preventing dirty gas from flowing from the dirty gas side into the interior of the chamber element; and a plurality of porous monolithic ceramic filter elements mounted in openings disposed in a side wall of the hollow chamber element, for allowing clean gas to flow from the dirty gas side into the interior volume of the hollow chamber element, and to the gas outlet.
The apparatus according to the present invention may advantageously utilize monolithic ceramic filter elements, such as CeraMem.TM. filters as shown in U.S. Pat. No. 5,114,581, or cross flow filters as shown in U.S. Pat. No. 5,078,760 (the disclosures of which are incorporated by reference herein). CeraMem.TM. filter elements are typically cylindrical having several parallel longitudinal passageways therethrough, the passageways being plugged to prevent dirty gas from flowing directly through the passageways and forcing clean gas to flow through the porous ceramic material into an adjacent passageway connected with the clean gas side. Cross flow ceramic filter elements are typically formed of several ceramic ribbed sheets forming crosswise clean and dirty gas channels.
According to a preferred embodiment of the present invention a filter supporting tube sheet is disposed in the upper part of an upright pressure vessel having a clean gas outlet at the top thereof, the tube sheet having supporting on the lower side thereof several parallel generally tubular candle like hollow chamber elements forming filter modules. The tubular elements have one open end and one closed end and are connected at their open end to openings in the tube sheet. The tubular elements further each having a row of monolithic ceramic filter elements mounted in openings disposed one on top of the other in a vertical side wall of the tubular elements. The monolithic filter elements are preferably all disposed on one and the same side of the tubular elements. The tubular elements may have a rather small diameter, as long as it is possible to insert the monolithic filter elements into openings made in the side walls of the tubular elements.
The porous ceramic filter elements, typically being cylindrical in form and having one inlet (dirty) end and one outlet (clean) end, preferably protrude with their outlet end relatively deep into the tubular elements, such that the ceramic element may cover&gt;1/2, or even more, of the cross sectional inner area of the tubular elements. The tubular elements need enough free inner space for clean gas to be able to flow through the elements into the clean gas side of the tube sheet.
The cross section of the tubular elements, i.e. the filter modules, may be substantially D-shaped, i.e. having one straight side and one generally semi-circular side. The ceramic filter elements are thereby easily connected to openings on the straight side. It is, of course, possible also to use tubular elements having polygonal (e.g. square) or circular cross-sections, and it is possible to dispose monolithic filter elements on all sides of the tubular elements. The tubular elements may also have partition walls therein. The filter elements may i.e. be arranged in, zig zag form, in turn on opposite sides of the tubular element, the filters overlapping each other within the tubular element.
When utilizing relatively thin tubular elements or filter modules it is easy to mount the tubular elements to the tube sheet. The present new tubular hollow chamber elements, being made of e.g. of metal plate material, cooled or non-cooled, are easy to connect to a tube sheet in conventional manner. Temperature differences, e.g. when backpulsing or when starting or shutting down a process, do not cause problems.
Also other advantages are achieved especially when backpulsing the ceramic filters, when the filter elements are disposed with their clean ends protruding rather deep, e.g.&gt;1/2 of them, into the tubular elements.
Backpulsing of monolithic ceramic filter elements in these narrow elongated tubular chamber elements can be done in a conventional manner e.g. as has been suggested for different candle type filters, by injecting clean gas (e.g. air) into the tubular chamber. The high pressure cleaning pulse thereby compresses the portions of the ceramic filter elements inside the tubular chamber from all sides, preventing mechanical breakage of the filter elements.
The tubular elements may be supported by their own constructions on the tube sheet. Alternatively several tubular elements may be supported by a common support structure connected to the tube sheet or the filtration vessel itself.
According to another embodiment of the present invention there may be only one or only a few hollow chamber elements, having a larger diameter than above mentioned tubular elements, within the filtration vessel. Such hollow chambers may have several vertical rows of ceramic filter elements disposed in openings on the side walls thereof. The filter elements may be disposed in groups, in vertical columns one on top of the other, in clusters vertically and horizontally aligned, or in circular rows around the whole periphery of the hollow chamber element, so that each group, i.e. vertical column, cluster or circular row, of ceramic elements may be connected to its own separate backpulsing system. Each separate backpulsing system may include a shielding forming a chamber-like element in front of each separate group of ceramic elements, the shieldings thereby also dividing the hollow chamber into separate clean gas zones. These zones are protected by the shielding when backpulsing other zones.
According to still another embodiment of the present invention there may be two or more tube sheets in the filtration vessels, two tube sheets e.g. dividing the inner space of a vessel into a middle dirty gas zone and a top and bottom clean gas zone. A dirty gas inlet is formed in the side wall of the vessel for leading dirty gas into the dirty gas zone. Clean gas outlets are formed between the tube sheets and the vessel top and bottom respectively. Several tubular elements with one single row of ceramic filter elements therein or larger hollow chamber elements with several rows of ceramic filter elements, may be disposed in the dirty gas zone between the tube sheets, connected at their open ends to either tube sheet. Tubular elements connected to the upper tube sheet, hanging downward from the sheet, and tubular elements connected to the lower tube sheet, protruding upwardly from the tube sheet, have a length preferably&lt;the distance between the tube sheets, the tubular elements thereby not interfering with each other. It is of course possible, if desired, to utilize longer tubular elements protruding from one tube sheet almost to the other, when there is enough horizontal space between the tubular elements coming from one side for the tubular elements from the other side therebetween. Also various baffles and separated particle guide means are provided to ensure optimum operational efficiency.
Cleaning of dirty gas having particles entrained therein may be performed using a vessel having at least one filter supporting plate or tube sheet dividing the vessel interior into a dirty gas side and a clean gas side, one or several vertical hollow chamber elements such as tubular elements being connected at their open end to an opening in the supporting plate and having several porous monolithic ceramic filter elements disposed in the side walls thereof. The cleaning process comprising the steps of: (a) Introducing high temperature gas with entrained particles to be filtered into the dirty gas side in vessel through an inlet opening in a side wall thereof. (b) Leading clean gas from the dirty gas side through the porous monolithic ceramic filter elements into the interior of the one or several hollow chamber elements and further to the clean gas side of the supporting plate for being discharged through a gas outlet arranged therein. (c) At predetermined intervals, directing a pulse of cleaning gas into the interior of the one or several hollow chamber elements, for backpulsing said porous monolithic ceramic filter elements; and (d) discharging solid particles dislodged from the ceramic filter elements through an outlet in the bottom of the vessel.
Dirty gas, such as process gas from a combustor or gasifier, introduced into the vessel may be guided by guide plates/baffles disposed in the vessel for dividing the gas as evenly as possible to all ceramic filters in the vessel. The dirty gas may be cleaned in ceramic filter elements mounted in vertical columns in tubular chamber elements, one column in each element. The tubular chambers are connected to a tube sheet in the vessel. Backpulsing gas may be introduced into each tubular element separately periodically or intermittently.